Transmission x-ray scanners used for personnel screening already exist in the market and are used in high-security areas where access is restricted from the general public, such as prisons, diamond and gold mines, and other places where small, high-value or dangerous items can be smuggled into or out from a secure area. One such system is described in U.S. Pat. No. 7,397,892 B2 to Linev, which issued on Jul. 8, 2008, and is incorporated herein in its entirety.
Linev teaches the use of an x-ray source that produces a single, fan-shaped x-ray beam that is collimated to produce a vertical beam of x-rays that is further collimated down to a very narrow slit. These collimated x-rays illuminate a single linear array of photo diodes coated with a scintillating phosphor. The person to be scanned stands on a motor-driven platform that moves the person slowly in between the source and the diode detector array exposing their entire body to the x-ray beam, thereby producing an x-ray image of their entire body. The x-ray image then reveals any objects they may have ingested, hidden in their clothing, or inserted in a body cavity.
The system taught by Linev, while effective because it can reveal the hidden items described above, suffers from a number of limitations. One of the primary limitations that the preferred embodiment of the Linev system suffers from is the inability to scan someone who has difficulty standing or is in a wheelchair. The platform (described in Linev as a walk-gate floor or movable door that is moving at constant speed) that is used to move the person being screened across the x-ray beam is small and difficult to access. It would, then, be a simple matter for a person to circumvent the scanner by claiming to need crutches, a walker, or a wheelchair. The scanning speed of the platform is necessarily slow to prevent the person standing on the platform from falling down or being injured. The slow scanning speed reduces throughput of the system and, thereby, the rate at which people can be scanned. Another limitation of this system is that x-ray radiation scattered from the person being scanned exposes anyone in the vicinity of the system to harmful radiation. This is because the system taught by Linev does not fully enclose and shield the walk-gate area. To mitigate this problem, a large exclusion area around the system must be established. This exclusion area greatly increases the amount of space required and increases the cost to install and operate the system. Any rooms adjacent to or in the floors above or below the system would also be similarly affected by this scattered radiation.
Yet another disadvantage of the system taught by Linev is a lack of control of the amount of radiation dose to which the person being scanned is exposed. The Linev system teaches the use of a fixed collimator and a detector positioning system. The exposure dose to the person being scanned is greatly affected by the accuracy in which the x-ray beam covers the detector array. If the width of the collimated fan beam of the x-ray source is larger than the width of the detector array, then x-rays that do not contribute to the image being formed are exposing the person being scanned, causing excess and unwarranted x-ray exposure. Linev also does not teach the use of varying the x-ray beam technique to optimize exposure parameters for each person being scanned. An x-ray beam technique refers to the x-ray energy (kVp), the integrated intensity (mAs), and the filtration used to acquire the image. If these x-ray exposure parameters are not adjusted to the specific body mass and anatomical region being scanned, then the exposure used to acquire the image is not optimal and, consequently, the dose used to acquire the image is not minimized. This could result in over-exposure or require a repeat exposure if the parameters are inadequate for an acceptable image (underexposure).
Yet another disadvantage of the system taught by Linev is the inability to create different configurations of the system that could provide flexibility in the installation and use of the system in different facilities. There are places, for example, such as office buildings, hotels, and private residences where the need for security exists but the physical presence of x-ray systems and equipment creates problems with available space and a desire to obscure or hide the security apparatus from view.
X-ray scanning systems already exist in the market and are used for whole-body imaging in applications like triage in trauma centers and emergency rooms in hospitals. One such system is described in U.S. Pat. No. 7,519,160 B2 to Vermeulen, issued on Apr. 14, 2009, which is incorporated herein in its entirety. Vermeulen teaches the use of a linear scanning x-ray apparatus that generates a collimated fan beam that is moved together with a detector relative to an object to generate a composite x-ray image of the subject. The x-ray generator and detector array is mounted on a C-arm that allows the generator and detector array to rotate about the patient being scanned, thereby obtaining a variety of views including chest and cross-table laterals.
The system of Vermeulen teaches an image acquisition mode where the x-ray source and detector array move together along an axis (the scanning direction) and a collimator creates a transverse fan beam of x-rays that illuminate a linear array of detectors. The divergent fan beam used by the linear scanning apparatus results in distortion of the image in a direction transverse to the scanning direction. In order to deal with the problem, the technique of x-ray computed tomography (CT) is used to correct for the distortion. This correction method is the subject of U.S. Pat. No. 7,873,142 B2 to Beets, issued on Jan. 18, 2011, which is incorporated herein in its entirety. Because the C-arm assembly holding the x-ray source and detector array has to be moved during a scan, the scanning speed must be kept at a slow rate to avoid significant hazards from collisions that could occur if the C-arm was moved at a high speed. Accordingly, the system requires up to 13 seconds to perform a whole-body scan of a patient.
Medical x-ray imaging systems require significantly higher spatial resolution than the x-ray imaging systems used in security applications. Accordingly, a significantly higher radiation dose is required to obtain a medical diagnostic image compared with an image obtained for security screening purposes. The spatial resolution used in security screening applications is about 0.5 line-pairs per millimeter (lp/mm) with an exposure dose of about 0.25 microSieverts. Medical diagnostic images require a spatial resolution of 2-4 lp/mm and require a dose of about 20 microSieverts. In order to minimize the exposure dose for medical applications when scanning systems are used, a technique called time delay and integration (TDI) is used. Special detector arrays that are designed for TDI image acquisition are currently available in the market today. One type is based on frame-transfer Charge Coupled Device (CCD) integrated circuit camera technology. Another type is based on a photodiode-CMOS integrated circuit TDI camara technology.
A linear array has a single row of detector elements that can be used in a scanning system to acquire an image by moving the source in the transverse direction of the linear array. In contrast, a TDI sensor array has several rows of detectors that are lined up in the direction of the scanning source. The image acquired by the TDI sensor is clocked out at a speed equal to the scanning speed in the opposite direction of the scanner. The result of this process is that the image signal acquired from each row of detectors is passed on to the next row of detectors where it is added to the next acquisition by superposition. If the clocking speed and scanning speed is equal and opposite in direction, the image signal is increased for each row of detectors. A TDI sensor with 128 rows will acquire an image with a signal intensity that is 128-times higher than a single linear array can produce. Alternatively, the scanner can scan at a higher velocity and acquire an image with less exposure dose. TDI image acquisition is taught by Vermeulen.